Hot plates are devices that provide a heated horizontal surface and are widely used in a variety of industrial and laboratory settings for heating substances contained in vessels. For instance, hot plates are commonly used for heating chemicals and other materials in open or closed vessels, in order to promote a chemical reaction or change in properties of the materials. A typical hot plate includes a heating element disposed below or embedded within a horizontal support surface for the vessel to be heated. A housing or base unit is usually provided for containing the electrical leads and connections between the heating element and an electrical supply line, as well as other components such as switches, over-temperature shut-off devices, potentiometer controls, and the like.
In many industrial and laboratory processes, there is frequently a need for the material in the vessel to experience motion or circulation simultaneously with it being heated. Thus, various types of stirring hot plates have been developed. One known type of stirring hot plate employs a magnetic stirring device which has a driving magnet mounted on the motor shaft directly below the support surface of the hot plate. The driving magnet produces a magnetic field that couples with a magnetic stirring bar placed in the material being heated, thereby causing the stirring bar to rotate in synchronism with the permanent magnet. By changing the speed and direction of rotation of the rotating drive magnet, the magnetically coupled stirring bar is effective to impart different types of stirring actions inside the vessel.
It is known to make hot plate tops from various materials, for example, copper iron, glass, aluminum and stainless steel; but all of those materials have disadvantages. For example, glass is breakable; and copper, iron and aluminum are subject to corrosion and/or oxidation from exposure to chemicals that may be present in the laboratory. Corrosion may be reduced by coating those materials, but such coatings are expensive and may not be commercially practical for less expensive hot plates.
Stainless steel has an advantage of being resistant to corrosion, but it has a disadvantage of being a relatively poor thermal conductor. Further, often a stainless steel top is connected to a base housing or unit at numerous points generally near a perimeter of the stainless steel top by a plurality of fasteners, soldering, welding, or other suitable connection. As the stainless steel top is heated, the heat is conducted unevenly through and across the stainless steel top resulting in variations in thermal expansion; and with the top tightly secured at its edges, the stainless steel top often buckles or crowns upward at its center. Thus, the varying temperatures and resulting varying thermal expansions produce a convex-shaped top surface, which reduces an area of contact between the top surface and the vessel being heated and substantially reduces the efficiency of the heating process. Further, the crowned or convex shaped top surface is more susceptible to the vessel moving or walking over the top surface in the presence of a vibration. If the hot plate has a stirring capability, the tendency of the vessel to walk is greater.
Consequently, there is a need for a hot plate that has a chemically resistant, stainless steel top, which, when heated, experiences minimal buckling or crowning.